Paper 1 Q1 — Central location: longest, not shortest

Cell biology — geometry of diffusion distance from a central organelle

What the examiners said

“Many candidates described the central location in the cell as being the shortest distance from the cell membrane for diffusion, when it is, in fact, the longest… Explanations as to how this distribution maximised efficiency were often incomplete.”

Try the question (reconstruction)

A carboxysome sits at the centre of an elongated cell. Compared with a position near the membrane, its distance for CO₂ to diffuse from the cell surface to it is:

Examiner report: “Candidates called the central location the shortest diffusion distance; it is the longest.”

Explore — diffusion distance from the membrane

Drag the carboxysome around the cell. The line shows the shortest path from the cell membrane to it — see how a central position gives the largest such distance.

Position along cell: centre

The efficiency argument in the exam therefore hinges on internal geometry (e.g. equal access from all cytoplasm), not on being close to the membrane.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H3 Biology 9816 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org